1. Field of the Invention
The present invention is related generally to polyamide based compositions. More particularly, the present invention is related to polyamic acid compositions and copolyimides derived therefrom having low thermal coefficient of expansion (TCE) and low stress while having lower anisotropy in thin film properties and improved polyimide-to-polyimide adhesion. This invention also relates to methods for forming multilayer metal/polyimide structures using low TCE copolyimide as an interlevel dielectric, insulator or passivation layer.
2. Description of the Prior Art
Polyimides have been widely used as high temperature polymer insulator/interlevel dielectric and passivation layer in microelectronics due to lower dielectric constant and other advantages in terms of process simplification and improved device performance in comparison to the commonly employed inorganic materials. The flexible chain polyimides that are derived from non-linear aromatic diamine and/or dianhydride precursors have relatively high thermal coefficient of expansion (TCE), typically in the range of 30-60ppm/.degree. C. compared to 2-3ppm in the case of inorganic materials (non-metallics) such as silicon, silicon oxide, silicon nitride, and ceramic substrates used in the fabrication of electronic device and packaging structures, and 6-25 ppm/C. in the case of commonly employed metallurgy. This TCE mismatch between the flexible chain polyimides and the contacting inorganic materials in electronic device and packaging multilayer structures results in the development of thermal stresses in the fabrication process or during stress testing that involve high temperature excursions often causing film cracking/delamination and thus presents a concern as to the performance reliability of the end product. Recently, due to better TCE match with contacting materials, the low TCE polyimides that are derived from linear-planar dianhydride and diamine precursors, typically, 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride (BPDA) and p-phenylenediamine (pPDA) have gained increasing importance as superior alternatives to the conventional polyimides carrying flexible chain segments such as PMDA-ODA which is derived from 1,2,4,5-pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA).
U.S. Pat. No. 4,690,999 is directed to certain low thermal expansion polyimides having restricted molecular mobility and the polyimide films may be uniaxially stretched to induce chain orientation. However, the patent does not disclose materials having the properties described in the present invention.
U.S. Pat. No. 5,115,090 is directed to BPDA-PDA and related polyamic acid compositions, and to low TCE polyimides derived therefrom, having certain thermal, mechanical, and passivation properties. However, the materials of the present invention are distinctive and different from those described in this patent.
It has been recognized in the art that the BPDA-PDA and related polyimides having rigid rod-like molecular chains have low in-plane thermal expansion due to in-plane orientation effects and strong interchain interactions which result in highly dense molecular matrix. It is generally known that the polyimide films formed from polyamic acid compositions derived from linear-planar dianhydrides and diamines such as BPDA and pPDA by spin coating and curing on silicon wafers or other inorganic substrates show large anisotropy, particularly as related to thermomechanical and optical properties compared to flexible chain polyimides.